There exists, throughout the world, a wide variety of mains AC voltages. Broadly, there are two groups: those of the low voltage range (88-132 Vrms) and those of the high voltage range (176-264 Vrms). It is desirable to design electrically powered equipment which can be used throughout the world, thereby to simplify design and production and to allow equipment to be portable between countries other than where originally purchased. This requires that the equipment must be adaptable for use on both voltage ranges.
The simplest way of achieving this has been to associate a manually operated doubler/bridge switch with the equipment. The operator will set the switch to the correct position before applying the mains voltage to the equipment.
FIGS. 1A and 1B illustrate a manually operated doubler/bridge switch. Four diodes D.sub.1 -D.sub.4 are connected together as bridge rectifier. An AC input voltage V.sub.LN is applied between nodes 2, 4, where node 2 is connected to the anode of D.sub.1 and the cathode of D.sub.2, and node 4 is connected to the anode of D.sub.3 and the cathode of D.sub.4. Two capacitors C.sub.1, C.sub.2, preferably of equal value, are connected in series between nodes 6, 8, being the DC output nodes of the bridge rectifier. A load L is connected between nodes 6, 8 and is supplied with the DC output. A switch T is connected between node 4 and node 10, being the node between the two capacitors. When the manual switch T is open, as shown in FIG. 1A, diodes D.sub.1 -D.sub.4 form a conventional bridge rectifier with capacitors C.sub.1, C.sub.2 to smooth the DC output generated. This is the bridge rectification mode. When the switch T is closed, as shown by FIG. 1B, diodes D.sub.1, D.sub.2 and capacitors C.sub.1, C.sub.2 operate as a combined voltage doubler and rectifier circuit of the known type. Diodes D.sub.3, D.sub.4 are superfluous in this mode. This is the voltage doubling rectification mode.
One major drawback of the manually operated doubler/bridge switch is that forgetting to check the position or choosing the wrong position of the switch would cause damage to the equipment, otherwise the equipment will have to be predesigned to prevent such damage, incurring extra cost.
An automatic doubler/bridge switch circuit is also known, which automatically doubles and rectifies a low range AC input voltage and which merely rectifies a high range AC input voltage. Sensing of the voltage level of the mains may be done by detecting peak voltage values of the AC waveform and using voltage comparators, or by sensing the magnitude of a DC rectified input voltage. Such circuits are available in either integrated circuit or discrete component form.
Such an integrated circuit automatic doubler/bridge switch is described in U.S. Pat. No. 5,162,984, of SGS-Thomson Microelectronics SA. A triac or equivalent thereof is used in place of the manual switch T.
A triac acts to conduct in either direction after the application of a negative (active low) pulse to its gate terminal, until the voltage across its two main terminals falls below a minimum value. By applying a gate pulse soon after every mains zero crossing point, the triac approximates to a short circuit. The introduction of such a short circuit provides the changeover from the bridge rectification mode shown in FIG. 1A to the voltage doubling rectification mode shown in FIG. 1B.
An alternating voltage is a voltage which undergoes successive upper and lower excursions, respectively more and less positive than a baseline voltage, such baseline voltage being the time average of the alternating voltage. For a mains alternating voltage, the baseline voltage is at the ground (zero volts) level.
The automatic voltage sensitive switch circuit described in the patent is a circuit containing several voltage comparators: these are used to detect peak voltage values to indicate a mains input AC voltage of the higher range, and to detect baseline crossing points to give timing information for triac gate pulses. Voltage reference sources are also built into the circuit to set the voltage reference points of the comparators. Means may be provided for generating many triac gate pulses per half-cycle of AC mains, to ensure efficient triggering in the presence of noise or disturbance on the AC lines. Filtering circuits may be included to remove such noise or disturbance from the voltage used for peak voltage detection.
Such integrated circuits are small and reliable. The cost of an automatic voltage sensitive switch circuit is reduced by reducing the number of components required, external to the integrated circuit.
Known automatic voltage sensitive switches employ a "single cycle" sensing mechanism, i.e. peak voltage level sensing is carried out on either only the upper or only the lower excursion of the mains voltage, linear extrapolation techniques being used to derive data for the other excursion.
This limitation is due to the fact that filtering, baseline crossing detection and peak voltage level sensing may only be carried out on a scaled representation of AC input voltages falling within a range limited by the two DC supply voltage levels of the control circuit of the automatic voltage sensitive switch. These are typically 0 V and 9 V. The AC input voltage is scaled down for input to the control circuit, but as it is centred on 0 V, the mains baseline, only one of the upper and lower excursions of the scaled AC voltage will fall within the allowed range for peak voltage level sensing.
Referring to FIG. 2, an embodiment of such an automatic voltage sensitive switch is shown. A doubler/bridge circuit 20 includes a triac T switching element; nodes 2, 4 are connected to the live L and neutral N lines of the AC mains, and receive the AC input voltage V.sub.LN. A diode D.sub.5, a resistor R.sub.4 and a capacitor C are respectively connected in series between the live line L and the neutral line N. Two resistors R.sub.1, R.sub.2 are connected in series between the live line L and the node 22 between the capacitor C and the resistor R.sub.4. The voltage between the neutral line N and node 22 is the voltage V.sub.C across the capacitor C. The automatic voltage sensitive switch control circuit 24 receives a first input voltage V.sub.SS from the node 22 and a second input voltage V.sub.4 from the neutral line N and a third, `composite`, input voltage, V.sub.M from the node 26 between resistors R.sub.1, R.sub.2, and supplies an output voltage V.sub.G T through a resistor R.sub.g to the gate G of the triac T.
Diode D.sub.5, resistor R.sub.4 and capacitor C form a half-wave rectifier and smoothing circuit and provide a DC supply voltage V.sub.C to the automatic voltage sensitive switch control circuit 24. V.sub.SS is taken as a ground voltage for the circuit. It is at a voltage V.sub.C less than the voltage of the neutral line N. (For example, 9 V less as fixed by a regulator within the control circuit). The AC input voltage V.sub.LN is applied to the potential divider comprising resistors R.sub.1, R.sub.2. The composite voltage V.sub.M received by the control circuit 24 is a scaled down AC input voltage, displaced by a small DC offset, given by: EQU V.sub.LN =V.sub.2 -V.sub.4,
where V.sub.2 and V.sub.4 denote the instantaneous voltages at nodes 2 and 4, respectively. ##EQU1## Using equation [1], ##EQU2##
Hence, both the scaled AC input voltage and the DC bias depend on the values of R.sub.1 and R.sub.2. Their values are chosen such that, for DC supplies of 0 V and 9 V to the control circuit, the peak voltage of V.sub.M will be less than 9 V in the presence of the highest possible instantaneous line voltage. For example, if R.sub.1 =1M.OMEGA. and R.sub.2 =18k.OMEGA., R.sub.2 /(R.sub.1 +R.sub.2)=0.0177, and the peak voltage for a 264 Vrms AC input would be 264.times..sqroot.2.times.0.0177=6.60 V. The input voltage to the control circuit V.sub.M would be an AC voltage of 6.60 V peak, almost centred on 0 V, but offset by 0.0177.times.V.sub.C =0.16 V. In this way, only the positive peak voltages of V.sub.M will fall within the allowed range of input voltages, fixed at V.sub.4 +0.6 V to V.sub.SS -0.6 V by input protection devices in the control circuit 24. The DC offset of 0.16 V means that the baseline of the composite voltage V.sub.M is at 0.16 V, and baseline crossing points may be detected, including hysteresis, by detecting when V.sub.M crosses 0.05 V and 0.25 V, for example.
FIGS. 3 and 4 show example input and output signals for the known automatic voltage switches carrying out comparison operations on the input voltage described. A scaled AC input voltage V.sub.IN is clipped by the input protection devices to remain within the range R, to obtain a measurable composite voltage V.sub.M. A pulse PD is produced when V.sub.M exceeds a certain threshold voltage V.sub.TP. Pulses CS represent the times during which V.sub.M &gt;0 V. The inverse of this signal is produced within the control circuit 24. A short pulse is produced at each rising edge of CS and its inverse, which are added to form the signal ZC, which indicates the baseline crossing points of V.sub.M, and hence the mains zero crossing points.
Peak detection pulses PD are produced at the peak of every upper excursion for a high range AC input voltage, using a comparator whose switching threshold is set to detect a voltage V.sub.TP in excess of the peak voltage of the highest scaled AC voltage of the lower range. For example, V.sub.TP may correspond to an instantaneous AC input voltage of 220 V, the peak value of a 156 Vrms AC input voltage. The threshold voltage must be carefully chosen, as AC mains voltages are subject to variation from time to time, resulting in the fact that the separation between a low range AC input voltage and a high range input voltage may be as small as 30 Vrms (140 Vrms to 170 Vrms).
Such peak detection pulses PD, disable the generation of triac pulses V.sub.G T.
As shown in FIG. 4, no peak detection pulses PD are produced for low voltage AC input as the composite voltage V.sub.M never passes the V.sub.TP threshold. As described above, a comparator whose switching threshold is the baseline voltage level produces the comparison signal CS. Its inverse is also produced, and short pulses are generated at each rising edge of both CS and its inverse. The sum of these short pulses is the mains zero crossing indication signal ZC.
Internal logic then uses pulses PD to extrapolate corresponding data for the other excursion, and to generate triac gate firing pulses V.sub.G T, a certain delay after each mains zero crossing point.
Voltage level detection is limited to one excursion only per cycle due to input protection devices, which limit the composite voltage V.sub.M to the allowed range R in FIGS. 3 and 4, being 0.6 V either side of the supply voltages. This eliminates any possibility of performing measurements on the other excursion per cycle of the waveform, in this example, the negative excursion.
Circuitry built into the control circuit 24 verifies a change in the apparent voltage range for at least two mains cycles before changing from bridge rectifier mode to voltage doubling rectification mode, so as to avoid the possibility of doubling a high range AC input voltage after a brief drop in AC input voltage.
A temporal filter is included within the control circuit to reduce the effects of glitches, sags, spikes and voltage surges. This is only active on input voltages which lie within the range R.
The limitation which is imposed on this circuit, in that it can perform filtering and voltage level sensing on only one half of the AC mains input signal results in several drawbacks.
Due to the voltage amplitude sensing on one half of the waveform only, there is the possibility of doubling a whole cycle of high voltage mains which may appear during an unsensed excursion of the AC input. Turn-off to non-doubling mode will not occur until the next baseline crossing after sensing has been performed on a sensed excursion of the input AC cycle. This could lead to damage to the bulk smoothing capacitors C.sub.1, C.sub.2 and the load L, due to one whole mains cycle of voltage far exceeding the device ratings, e.g. 480 V DC, being applied, as the DC output voltage will become double the AC input voltage for one whole cycle.
Furthermore, the filtering described is not effective on half the wave, leaving the control circuit vulnerable to damage caused by such transients on the unsensed portion of the AC input wave. To date, these deficiencies have been tolerated.
Automatic voltage switches using the detection of a DC rectified version of the AC input voltage are also known, for example as described in U.S. Pat. No. 4,665,323, but these have the disadvantage of requiring additional high voltage diodes, smoothing capacitors and power resistors, which cannot be included in an integrated circuit, and therefore leading to a complex and expensive solution.
Automatic voltage switching circuits also exist in discrete form, whereby the circuit contains a voltage comparator, with one input referenced to a zener diode, to detect rectified or scaled AC voltages above a certain upper or lower threshold, as defined by the zener diode, and circuitry to provide triac gate firing pulses when required.
The object of the current invention is to provide a DC voltage supply circuit for rectifying an alternating voltage, using a minimum of discrete components, whilst improving sensing accuracy and reducing the circuit cost and complexity.